1. Field of the Invention
The present invention relates generally to devices for positioning one optical element relative to another optical element in an optical instrument, wherein selective relative movement between the optical elements is necessary for measurement purposes. More particularly, the present invention relates to a device for repeatably positioning a pair of optical surfaces, one from each optical element, opposite each other to consistently replicate an optical path in the instrument. The present invention is applicable, for example, to the field of refractometry and refractometers.
2. Description of the Related Art
Optical instruments for performing test measurements often require that an optical path be established that includes a test subject or sample in the optical path, whereby an effect of the test subject or sample on light traveling along the optical path can be observed. An important factor in the precision of such instruments (that is, the ability of the instrument to provide a reproducible measurement) is the consistency with which the optical elements defining the optical path are located relative to the test subject or sample and one another, especially where one or more optical elements are selectively movable temporarily away from a measurement position to allow the test subject or sample to be introduced in the optical path.
An example of this type of optical instrument is a transmitted light refractometer, such as an Abbe or Pulfrich refractometer used for measuring the refractive index of liquids. Transmitted light refractometers are known to include a prism assembly comprising an illumination prism cooperating with a sample prism. The sample prism has a sample surface for receiving a transparent or translucent test sample, and the illumination prism directs light from a light source to be obliquely incident at a known angle to the test sample, whereby the critical angle of total reflection at the interface between the sample and sample surface of the sample prism is directly or indirectly observed. Consequently, it is necessary for a light exit surface of the illumination prism to evenly contact the test sample.
In prior art refractometers of the type described above, the illumination prism is typically mounted in a hinged illumination prism carrier that opens pivotally to allow the sample to be placed on the sample surface and closes pivotally over the sample. A biasing load is applied by mechanical means or simple gravity to ideally provide a uniformly thin layer of sample liquid between the exit surface of the illumination prism and the sample surface of the sample prism, with the exit and sample surfaces ideally orientated parallel to one another.
U.S. Pat. No. 4,650,323 discloses a hand-held refractometer that includes a weight 29 on a movable plate 23 provided in the illumination prism carrier to apply a uniform vertical load to the sample liquid. See column 5, lines 36-47. This arrangement is well suited for a hand-held refractometer with relatively low precision and accuracy demands. However, play in the hinge mechanism and instrument tilt angle at the time of reading cause variations in the magnitude and line of action of the weight-induced biasing force that tends to bring the exit surface and sample surface together, which variations result in a wider scattering of measurement data for a series of measurements taken with respect to a constant test sample than is desired for applications where accuracy and precision demands are relatively high.
Heretofore, manufacturers of Abbe refractometers, such as Atago, Bellingham+Stanley, Spectronic Instruments, Milton Roy, and Leica Microsystems Inc. (assignee of the present invention) have relied on mechanical clips, friction locks, and cam locks operating between the illumination prism carrier and sample prism housing to apply a biasing force tending to bring the exit surface of the illumination prism and the sample surface of the sample prism together. However, there are limitations to such mechanical devices which have a detrimental effect on measurement accuracy and precision. First, the force is not arranged symmetrically with respect to the prism surfaces, whereby non-uniform sample thickness occurs. Second, the mechanical devices do not correct for small variations in the locational alignment of the exit surface relative to the sample surface due to play in the hinge mechanism. Third, mechanical devices with moving parts are subject to wear over time. In part because of these limitations, automatic Abbe refractometers of the prior art have not exceeded an accuracy on the order of +/xe2x88x920.0001 for refractive index readings.
Therefore, it is an object of the present invention to provide a device for positioning one optical surface relative to another optical surface in an optical instrument in a repeatable or reproducible manner.
It is another object of the present invention to provide a device that achieves reproducible positioning of optical surfaces without the use of mechanical clips, friction locks, cam, locks, or weights.
It is a further object of the present invention to provide a device that positions one optical surface closely adjacent and parallel to another optical surface by establishing a constant and symmetrically applied biasing force tending to bring the optical surfaces together, whereby a thin uniform optical layer can be achieved between the surfaces.
It is a further object of the present invention to provide a device that self-aligns and locates one optical surface relative to another optical surface.
It is a further object of the present invention to provide an improved prism assembly for refractometers that is easy to assemble and clean.
Pursuant to these and other objects, the present invention is embodied in a prism assembly of a refractometer having an illumination prism mounted for selective movement to and from a measurement position with respect to a sample prism. The sample prism is fixed in a housing and includes an exposed sample surface for receiving a liquid sample, and the illumination prism is mounted in a carrier hingedly connected to the refractometer such that a light exit surface of the illumination prism can be selectively positioned directly opposite and closely adjacent the sample surface. In a preferred embodiment, a plurality of magnets is set within recesses in the illumination prism carrier to be flush with an engagement surface of the carrier. The magnets are symmetrically arranged about the length and width of the rectangular exit surface. Likewise, a plurality of corresponding magnets is set within recesses in the sample prism housing carrier to be flush with an engagement surface of the housing. Each magnet in the carrier matches up and cooperates in attraction with a respective magnet in the housing when the prism assembly is in its measurement position.
The present invention improves instrument accuracy and precision by creating a uniformly thick layer of sample fluid between the light exit and sample optical surfaces due to a constant and repeatable biasing force urging the optical surfaces together. The magnets also serve to self-align the surfaces relative to one another to minimize detrimental misalignment due to play in the hinge mechanism for the carrier.